The invention relates to a process and a device for air separation by low temperature rectification of air, in which air is compressed, prepurified, cooled, fed to a two-stage rectification and separated into an oxygen-rich and a nitrogen-rich fraction and, from the low pressure step of the rectification, another oxygen fraction enriched with argon is removed and separated in a crude argon rectification into crude argon and into a higher boiling residual fraction.
The main products of an air separation, oxygen and nitrogen, can be removed directly from the two-stage rectification. Argon, on the other hand, whose boiling temperature is between the boiling temperatures of oxygen and nitrogen, becomes enriched in the middle section of the low pressure stage of the rectification. At this point, a fraction of mostly oxygen is removed, but in this fraction a large part of the argon contained in the air feedstream is drawn off. This fraction is separated by rectification in a crude argon column into crude argon and a liquid residual fraction. The residual fraction is fed back into the low pressure step.
A process of the type mentioned above is known from DE-OS-34 36 897. There, following a two-stage air rectification in a crude argon column, gaseous crude argon is extracted that contains up to about 95% argon and is contaminated mainly by about 3% oxygen and 2% nitrogen (all percentages refer to the volume). In the previously known processes, during rectification in the crude argon column, which usually contains about 60 exchange plates, the oxygen can be only incompletely removed, since the boiling point of argon and oxygen are extraordinarily close to one another. The difference in the boiling temperatures is, for example, 2.9 K. at a pressure of 1 bar.
If pure argon containing less than 1% impurities is to be extracted, then the remaining oxygen, which exhibits a slightly higher boiling point than argon, must be removed from the crude argon extracted in the known way, before the lower boiling nitrogen is separated in a pure argon column by rectification.
The separation of the oxygen from the crude argon is performed in the known processes in a so-called deoxo device in that the oxygen is burnt with hydrogen mixed in and the water resulting in doing so is separated in a dryer. Such a process has been disclosed, for example, in DE-OS 34 28 968.
Such a deoxo device represents an expensive apparatus and causes, above all, high operating costs due to the not inconsiderable consumption of hydrogen. Especially expensive is the preparation of the hydrogen if it is not readily available from chemical processes that are performed at the site of the air separation unit.